The present invention relates to a switching apparatus having MOS transistors, as is used, for example, to control motors, to control relays, to control lamps, for DC/DC conversion or to drive other inductive or resistive loads. As a result of their switching, such switch apparatuses produce interfering conducted and non-conducted emissions. Emissions, whether conducted or non-conducted, are generally undesirable since they may result in interference in a system. Emissions may be reduced by virtue of the MOS transistor switch carrying out the switching operation as slowly as possible. Natural limits which result from the application are imposed on slow switching. Such a limit is if the duration of the switching operation becomes too long in comparison with the duration of the switching period. Another limit is if the losses of the switching operation are greater than the losses of a switch in the switched-on state. In addition to slow switching, a constant current profile is needed to successfully reduce interference emission.
In order to suppress the remaining interference emission, filters must be used at suitable locations. Abrupt transitions in the current flow contribute to the emission. In this case, the greatest temporal change which can be achieved in the case of MOS power transistors occurs in the vicinity of threshold voltage. The apparent switching operation begins on the basis of the transfer characteristic of the power transistor, its temperature, its threshold voltage, the current density set by the load, in the event of a different voltage of the control electrode and with respect to the necessary charge on the control electrode with a fixed charging current at different times.
The problem of low-radiation switching-on has previously been solved by comparing the switched load current of the MOS transistor with a reference, for example. If the load current of the MOS transistor is less than that of this reference, the switching-on operation is slowed down. After the load current of the MOS transistor has been exceeded, the desired higher transconductance is set. Determining a particular load current of the MOS transistor as a reference value is disadvantageous in this case. This results in discontinuity in the switch current and a great variation in the delay of the switching-on operation. It is not possible to minimize the delay of the switching-on operation since a changeover operation is not carried out here until current flow has already begun. The actual start of the switching operation remains unknown in this case.
Another possibility is to charge the control connection of the MOS transistor with a linearly rising current. In this case, the delay time before the true start of the switching-on operation and the initial transconductance of the MOS transistor may vary to a very significant extent. When designing this linearly rising current for driving the MOS transistor, the load current of the application should be known. In addition, it should be taken into account that many parameters influence the start of the switching-on operation and its fluctuation.
DE 19640433 exhibits a switching apparatus having a MOS transistor for reducing the emission during the switching-off operation. FIG. 2 illustrates a zener diode chain having means which are suitable for delaying the build-up of the zener voltage so that the edges are rounded off, which reduces the emission of the switching-off operation.
DE 10000020 exhibits a circuit arrangement which is designed to drive a MOS transistor with a delay time.
DE 10061371 exhibits a circuit arrangement for driving a load, in which a semiconductor switching element having a protection circuit, in particular having an overtemperature protection circuit or a protection circuit for current limitation, can be used. The semiconductor switching element is also protected against an overvoltage on its load path.
DE 10245046 exhibits a circuit arrangement which is designed to dynamically change the commutation voltage during the commutation operation.
DE 4029794 exhibits a method for driving an electromagnetic load and a device for driving an electromagnetic load.
EP 1071213 exhibits an integrated circuit for driving a power MOS transistor with means for detecting the presence of an external load in order to optimize the switching operation of the MOS transistor.